Recently, a technique for producing a semiconductor device, in which semiconductor thin films are formed on an inexpensive glass substrate, such as a thin film transistor (TFT), has been quickly developed. This is because there is an increasing demand for an active matrix liquid crystal display device (liquid crystal panel).
An active matrix liquid crystal panel is composed of several tens to several millions of pixel areas, at each of which a TFT is arranged, and electric charges going in and out the respective pixel electrodes are controlled by a switching function of the TFTs.
A conventional analogue gradation type active matrix liquid crystal display device is shown in FIG. 19. The conventional active matrix liquid crystal display device has a driver on a source line side 2001, a driver on a gate line side 2002, plural pixel TFTs 2003 disposed in a matrix form, and image signal lines 2004, as shown in FIG. 19.
The driver on a source line side and the driver on a gate line side each contains a shift register and a buffer circuit, which are recently integrated on the same substrate as the active matrix circuit.
In the active matrix circuit, thin film transistors utilizing amorphous silicon formed on a glass substrate are arranged.
It has been known to produce a thin film transistor using a polycrystalline silicon film on quartz as a substrate. In this case, peripheral driver circuits and an active matrix circuit are produced with thin film transistors formed on a quartz substrate.
It has been also known to produce a thin film transistor using a crystalline silicon film on a glass substrate utilizing a laser annealing technique. By using this technique, an active matrix circuit and peripheral driver circuits can be integrated on a glass substrate.
In the constitution shown in FIG. 19, image signals supplied to the image signal line 2004 are selected by signals from a shift register circuit of the driver on a source line side (shift register for horizontal scanning), and the prescribed image signals are supplied to the corresponding source signal line.
The image signals supplied to the source signal line are selected by a thin film transistor of a pixel and written in the prescribed pixel electrode.
The thin film transistor of the pixel is driven by selection signals supplied from a shift register circuit of the driver on a gate signal line side (shift register for vertical scanning) via the gate signal lines.
These operations are repeated one after another at suitable timing with the signals from the shift register of the driver on a source signal line side and the signals from the shift register of the driver on a gate signal line side, and the signals of an image are written in the respective pixels disposed in a matrix form.
An active matrix liquid crystal display device is frequently used in a portable personal computer in recent years. In a personal computer, because operations are often conducted where plural softwares are simultaneously operated and an image is imported from a digital camera, a liquid crystal display of multiple gradation is required.
A demand of a liquid crystal projection display capable of displaying a large scale picture is being increased. The image quality of such a display depends on the fineness of gradation display and the fastness of signal processing.
The methods of gradation display include the case where analogue signals such as video signals and television signals are supplied to the source lines (analogue gradation) and the case where digital signals such as those from a personal computer are supplied (digital gradation).
In the case of analogue gradation as described above, the analogue image signals supplied to the image signal line are selected one after another by signals from the source driver to supply the prescribed image signal is supplied to the corresponding source line.
In the case of digital gradation, the digital signals supplied to the image signal line are selected one after another, and after subjected to digital/analogue conversion, the prescribed image signal is supplied to the corresponding source line.
In any case of gradation display, liquid crystal display devices have the relationship between the voltage (V) applied to the liquid crystal and the transmission light intensity shown as the dotted line in FIG. 20. The liquid crystal display used herein is one of a normally white mode, which transmits light when a voltage is not applied in TN (twisted nematic) mode.
It is understood from FIG. 20 that there is a non-linear relationship between a voltage applied to the liquid crystal and the transmission light intensity, and it is difficult to display gradation corresponding to the applied voltage.
Gamma correction is employed in order to compensate the difficulty. In gamma correction, the image signals are gained to correct so that the transmission light intensity is linearly changed corresponding to the applied voltage, by which good gradation display is obtained. The relationship between the applied voltage and the transmission light intensity is shown as the solid line in FIG. 20.
In order to make gamma correction on the image signals, however, an IC chip equipped with a signal processing circuit and a memory circuit has been required. Furthermore, in order to display a large scale picture, another correction circuit and signal processing circuit, as well as a memory circuit associated with them, have been required. The signal processing circuit and the memory circuit have been provided as an IC chip equipped outside the liquid crystal display panel. Accordingly, the miniaturization of commercial products has been practically impossible.